method for producing a dental restoration

ABSTRACT

A method for preparing a dental restoration with at least one rotating material removing tool ( 2 ) is presented, the N method comprising the steps of providing a dental material piece ( 1 ) from which the dental restoration is to be prepared, providing an initial cavity (C) in the material dental piece ( 1 ), and removing material outside the initial cavity (C) by moving the tool ( 2 ) essentially in a plane perpendicular to the rotational axis (R) of the tool ( 2 ). In hard, brittle dental restoration materials, risks of material failure, excessive tool wear, and tool failure are reduced, and higher processing speeds are made possible.

TECHNICAL FIELD

The present invention relates to a method for preparing a dentalrestoration with at least one rotating material removing tool.

BACKGROUND

Since the 1980:s developments of automated production of dentalrestorations have been made. Such a production typically includeautomated acquiring of topographic data from a model made from a biteimpression from a dental patient, computer based design of a dentalrestoration, and automated manufacturing of the dental restoration. Forexample, CAD/CAM based systems from the design and manufacturing ofdental restorations are known from:

-   -   Duret: “Vers unit prothese informatisee” Tonus Dentaire No 73,        1985 pp. 55-57.    -   Duret et al: “CAD-CAM in dentistry”, JADA, Vol. 117, November        1988, pp. 715-720.    -   Williams: “Dentistry and CAD/CAM: Another French Revolution”,        Journal of Dental Practice Administration, January/March 1987.    -   Sjolin, Sundh, Bergman: “The Decim System for Production of        Dental restorations”, International Journal of computerised        Dentistry 1999: 3.

In an automated manufacturing of a dental restoration, typicallysuitable tools, such as cutting tools, are used to form the restorationfrom a blank, the tools following paths according to a manufacturingprogram based on a digital model of the dental restoration. Usually,industrial ceramics, such as dense sintered high purity aluminium oryttrium stabilized zirconium, are used as material for the restoration.Such materials present, despite their advantages concerning theesthetical result of the restoration, a number of problems in themanufacturing process. Their hardness result in a high rate of wear onthe tools used, which, besides being costly, can result in vibrations inthe manufacturing process, in turn causing deviations from tolerancerequirements. Also, the nature of the ceramic materials used is suchthat they are relatively brittle, and therefore caution has to be takenwhen the tool paths are determined, in order not to avoid the risk offailure in the material. Usually, restriction on the cutting parametersof the tools during the manufacturing process are introduced to decreasetool wear and avoid failure in the blank, which in turn lengthens theprocess causing a slow production.

SUMMARY

It is an object of the invention to decrease tool wear in automatedmanufacturing of dental restorations.

It is another object of the invention to decrease, in automatedmanufacturing of dental restorations, the risk of a failure in thedental restoration material.

It is another object of the invention to decrease processing time inautomated manufacturing of dental restorations.

These objects are reached with a method for preparing a dentalrestoration with at least one rotating material removing tool,comprising the steps of

-   -   providing a dental material piece from which the dental        restoration is to be prepared,    -   providing an initial cavity in the material dental piece, and    -   removing material outside the initial cavity by moving the tool        essentially in a plane perpendicular to the rotational axis of        the tool.

In brittle dental materials, to reduce risks of material failure, theworking grinding surface of the tool should have a high speed inrelation to the material. The speed of the grinding surface of the tooldue to the rotation of the latter, is zero at the axis of rotation,which is usually at the tip of the tool. When the tool is moved in aplane perpendicular to the rotational axis of the tool, the area on thegrinding surface of the tool having no or little speed will moveparallel to the surface of the dental material piece. Therefore, thisarea will not be substantially involved in the material removal process.Instead, areas of the tool further from the axis of rotation, having ahigh speed will be involved in the process. Therefore, not only risks ofmaterial failure will be reduced, but also, due to the high speed ofworking grinding surfaces, the tool can be moved at a higher speed,which shortens the processing time of the dental restoration. It isknown that dental restoration materials causes a lot of wear on materialremoval tools. An advantage of the invention is that the high speed ofworking surfaces of the tool will reduce wear of the tool itself.

When forming a cavity in a dental material using the method according tothe invention, movements of the tool in a direction having a componentparallel to the rotational axis, causing low speed areas of the tool totake part of the material removing process, can be limited andconcentrated to a step of forming an initial, central cavity, and asubstantial part of the material removal procedure can be performed bymoving the tool perpendicular to the rotational axis.

Preferably, the step of providing an initial cavity includes moving thetool so that the direction of the movement of the tool forms an angle toa rotational axis of the tool.

At a distance from the center of rotation, the surface of the tool has avelocity component due to the tool rotation in a direction which istangential to the local work piece surface. However, a surface area ofthe tool close to or at the center of rotation has only a small velocitycomponent or no velocity component due to the rotation of the tool inthe tangential direction of the local surface of the work piece. Bymoving the tool in an angle to the rotational axis of the tool, such anarea close to or at the center of rotation will present a velocitycomponent, due to the translational movement of the tool, in thetangential direction of the local surface of the work piece. Thereby,the temperature buildup, and the risk of excessive tool wear and workpiece material failure is substantially decreased. Also, since thetemperature buildup, and the risk of excessive tool wear and work piecematerial failure is decreased, the tool can be allowed to removematerial at a higher rate.

Also, since the tool is moved in an angle to the rotational axis of thetool, it is assured that an open space will be present close to a tip ofthe tool, providing for a cooling liquid to be distributed to an areaclose to the effective working area of the tool.

Preferably, the angle between tool movement direction and the rotationalaxis of the tool is between 80 and 89.5 degrees. Within this range ahigh processing speed is allowed with risks of material failure keptlow. More specifically, while moving the tool at about 200 mm/min., forrelatively hard dental restoration materials a suitable value for saidangle is around 89 degrees, and said angle can be decreased to about 85degrees when working in softer dental restoration materials, giving ahigh processing speed with little risk of material failure, excessivetool wear or tool failure. Examples of hard dental restoration materialsinclude aluminium oxides and fully sintered yttrium stabilisedzirconiumdioxide, and the exceptionally hard, hot isostatic pressedzirconiumdioxide. Relatively soft dental restoration materials includemagnesium stabilised zirconiumdioxides.

Preferably, in the step of providing an initial cavity, the tool path,as projected in a plane perpendicular to the rotational axis of thetool, forms a closed loop. Thereby, the tool path could be helical, orpresent a screw form having an elliptic, rectangular, square, ortriangular cross-section. Alternatively, the tool path, as projected ina plane perpendicular to the rotational axis of the tool, could presenta closed curve of any suitable, alternative shape. By letting the tooldescend into the work material while the tool movement as projected in aplane perpendicular to the tool rotational axis forms a closed loop, thesize of the tool surface in contact with the work piece, can becontrolled so that it does not exceed a desired level, and is kept, atleast substantially, constant.

Preferably, the method comprises

-   -   determining a tool center boundary curve, which represents the        outer limit of the movements of the rotational axis of the tool,        at a plane perpendicular to the axis of rotation of the tool,        based on    -   the intended final cavity surface in a region in the vicinity of        an intersection between said intended final cavity surface and        said plane perpendicular to the axis of rotation of the tool,        and    -   the shape of the tool on at least a part thereof.

When performing the step of removing material outside the initialcavity, material is removed essentially until an intended final cavitysurface of the dental material piece. However, to arrive at the finalshape, precision machining has to be performed to smoothen the surfaceof the dental material piece. This is a time consuming stage of theprocess of obtaining a dental restoration. By considering, in apreceding stage, the shape of the tool in relation to the intended finalcavity surface, the result of such a preceding stage will come closer tothe end result. In turn, less material will remain to be removed in afollowing precision machining stage, and less time will be involved inthe latter, contributing to shortening the entire dental restorationproduction process.

According to a preferred embodiment, the method comprises

-   -   determining a tool center curve at each level, of a plurality of        levels, at, under and/or above said plane being perpendicular to        the axis of rotation of the tool, by offsetting inwards the        intersection between the intended final cavity surface and the        respective level by an amount corresponding to the radius of the        tool at the respective level, and    -   determining the tool center boundary curve as the most inwardly        located of the tool center curves.

This provides a two dimensional calculation at each level, rendering a2½ dimensional calculation for determining the tool center boundarycurve, giving a result, essentially as accurate as a three dimensionalcalculation. However, compared to the latter, considerably lesscalculation steps are involved in the preferred embodiment of theinventive method. Therefore, the calculation time, and thereforeprocessing time of the dental restoration can be kept low, in additionto which the method can be performed at a dental technician laboratoryhaving limitations regarding the computational capacity of its computerequipment.

Preferably, the location, in a plane perpendicular to the rotationalaxis of the tool, of a center of the initial cavity is determined as thelocation of the center of the largest circle that can be fitted within aboundary curve in a plane perpendicular to the axis of rotation of thetool. Thereby, the step of removing material outside the initial cavityis advantageously performed by moving the tool, from the initial cavitytowards the boundary curve, along circular paths or a spiral shapedpath. Determining the largest circle, that can be fitted within aboundary curve in a plane perpendicular to the axis of rotation of thetool, has the result that the length of circular paths or a spiral pathinside the largest circle is maximized. In turn, this is advantageoussince the effective grinding area of the tool can be controlled andvariations in the effective grinding area can be kept at a minimum.

Preferably, at least one tool path is determined by creating at leastone offset curve by offsetting outwards a curve, and trimming the atleast one offset curve against a tool boundary curve. As will beexplained further below, this has the advantages that the cutting depthof the tool can be controlled, and that it is easy to check if the toolpaths result in extraordinary movements that are undesired from amaterial processing point of view, e.g. due to a risk of damaging thematerial or the tool.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages of the invention will be presented in thedescription below, in which the invention will be described in detailwith the aid of the drawings, in which

FIGS. 1, 4, and 9-17 show cross-sections of a dental material piece fromwhich a dental restoration is to be prepared, in different stages of amethod according to one embodiment of the invention,

FIG. 2 shows a perspective view depicting a tool and its movement,

FIG. 3 shows a side view of the tool in FIG. 2 in action,

FIG. 5 shows a view of a detail of the tool and a detail of the dentalmaterial piece,

FIG. 6 depicts tool boundary curves,

FIG. 7 shows a sectioned view of the dental material piece, whereby thesection is oriented perpendicular to a rotational axis of the tool, and

FIGS. 8 a, 8 b, and 8 c show tool paths projected in a planeperpendicular to a rotational axis of the tool.

DETAILED DESCRIPTION

FIG. 1 shows a cross-section of a dental material piece 1 from which thedental restoration is to be prepared. The dental restoration could be acrown, a part-crown, an inlay, an onlay, a bridge, a stumpreconstruction, a veneer, also referred to as a ligament, a facette, afilling or a connector. The dental restoration could be formed accordingto a digital model, in turn obtained by scanning of a model, obtainedfrom a bite impression, and a computer aided design process based on thescanning data, known in the art. The dental material piece 1 could be ablank, or the result of an already initiated material removal process ona blank. For example, an exterior surface of the dental restorationcould be at least partly finished, before commencing the steps of themethod according to the invention.

The dental material of the piece 1 could be a ceramic material based onzirconium oxide, aluminium oxide or any other suitable material. Thedental material piece 1 is mounted in a machine with at least one holder(not shown in FIG. 1).

In the machine any suitable material removing tool 2 can be arranged,such as a milling tool or a cutting tool, suitable for working on thematerial for the restoration, whereby the tool is adapted to moveautomatically in relation to the dental material piece 1 according toinstructions in a program file run in a computer program. The rotationalaxis of the tool 2 is indicated in FIG. 1 with a line R. The toolpresents a cylindrical grinding surface 4, and an essentially flatgrinding surface 3′ at the tip region 3, which flat grinding surface isoriented essentially perpendicular to the rotational axis R. A radius 5is provided at the intersection of the flat grinding surface 3′ and thecylindrical grinding surface 4. Alternatively, the grinding part of thetool could have another shape, e.g. of a truncated cone or a sphere.

The tool is to be used in a process of removing material to obtain acavity of the dental restoration. In FIG. 1 the dental material piece 1is shown sectioned parallel to the rotational axis R of the tool 2. Acontour of the intended cavity is indicated with the broken line 6.

Referring to FIG. 2, in a step according to a preferred embodiment ofthe invention, an initial cavity is formed in the dental material piece,by moving the tool 2 while in rotation, wherein the tool follows ahelical path. In FIG. 2, the helical path is indicated as a pathfollowed by a center of the tool 2, and indicated by a curved arrow P.Thus, the path P forms an imaginary screw. To avoid material remainingat the center of the bore formed by the tool, the diameter of this screwis less than the diameter of the tool itself.

The helical path described results in the effective working surface ofthe tool being substantially constant during this step of the method.However, as an alternative to the helical motion described, it ispossible to move the tool along another descending path with a differentshape when projected in a plane perpendicular to the rotational axis Rof the tool. Thus, the shape of the path projected in a planeperpendicular to the rotational axis R of the tool could be elliptic,rectangular, square or triangular. Alternatively, the path P formed inthis step of the method is not closed when projected in a planeperpendicular to the rotational axis R, whereby it is simply a curved orstraight declining path.

Referring to FIG. 3, during the step described with reference to FIG. 2,the tool 2 is moved so that the direction of the movement P forms anangle α to the rotational axis R of the tool 2. If the tool is moved ata velocity of about 200 min/min, the angle α is suitably about 89degrees for hard dental restoration materials, and down to 85 degreesfor less hard dental restoration materials. Thereby, the materialremoval rate can be kept relatively high, at the same time avoiding therisk of material failure, excessive tool wear or tool failure due tohigh temperatures in the effective grinding region.

FIG. 4 shows the result of the step described above with reference toFIGS. 2 and 3. An initial substantially cylindrical central cavity C hasbeen formed with a diameter essentially equal to the diameter of thescrew of the helical path P added to the tool diameter. Material hasbeen removed from a first level L1 of the dental material piece 1 to asecond level L2 thereof, the first and the second level L1, L2 beingseparated by a distance d2 in a direction parallel to the rotationalaxis of the tool. Here the expression “level” means an imaginary flatplane perpendicular to the rotational axis R.

Referring to FIGS. 5 and 6, at the second level L2, a tool centerboundary curve TCBC is determined, which represents the outer limit ofthe movements of the rotational axis R of the tool, at the second levelL2. The determination of the tool center boundary curve TCBC is based onthe intended final cavity surface 6 (FIG. 4) in a region in the vicinityof an intersection between said intended final cavity surface 6 and thesecond level L2, and also the shape of the tool.

Referring to FIG. 5, more specifically a tool center curve TCi, TCi-1,TCi-2, TCi-3 is i-2, i-3. The levels, i, i-1, i-2, i-3, which can be ofany suitable number, can be located at, under and/or above the level L2,but in this example, one level, i, is identical to the second level L2,and the remaining levels, i-1, i-2, i-3, is distributed above the secondlevel L2. At each level, i, i-1, i-2, i-3, a line formed by theintersection between the intended final cavity surface 6 and therespective level i, i-1, i-2, i-3 is offset inwards by an amountcorresponding to the radius Ri, Ri-1, Ri-2, Ri-3 of the tool 2 at therespective level, i, i-1, i-2, i-3, whereby a tool center curve, TCi,TCi-1, TCi-2, TCi-3, is determined at each level, i, i-1, i-2, i-3.

Referring to FIG. 6, the tool center boundary curve TCBC, indicated inFIG. 6 with a bold line, is determined as the most inwardly located ateach segment of the tool center curves, TCi, TCi-1, TCi-2, TCi-3.

Preferably, a step of removing material outside the initial cavity C bymoving the tool 2 essentially in a plane perpendicular to the rotationalaxis R of the tool 2, includes moving the tool 2 along concentriccircular paths. To provide for obtaining a maximum length of suchcircular paths, a center H (see FIG. 4) of the initial cavity Cdescribed above with reference to FIGS. 2 and 3 is determined in thefollowing way:

Referring to FIG. 6, the center H of the initial cavity C is determinedas the location of the center of the largest circle C9 that can befitted within a the tool center boundary curve TCBC. Alternatively, thecenter H of the initial cavity C can be determined as the location ofthe center of the largest circle that can be fitted within some otherboundary curve, for example, the intersection between the second levelL2 and the intended cavity surface 6, (see FIG. 4).

Thus, the center of this circle C9 is the lateral position of the centerof the screw formed by the helical path P described above with referenceto FIG. 2. Accordingly, preferably, the tool center boundary curve TCBCat the second level L2 is determined before creating the initial cavityC.

Following the step of providing an initial cavity C, material is removedbetween the first and the second level L1, L2 by moving the tool 2 whilein rotation essentially in a plane perpendicular to the rotational axisR of the tool 2. Material is to be removed approximately until theintended cavity surface at the second level L2.

FIG. 7 shows, in a cross-section perpendicular to the rotational axis ofthe tool, a step following the step of providing an initial cavity. Themovements of a center position of the tool 2 at the rotational axis Rthereof, are indicated with lines with arrows.

The movements have directions essentially perpendicular to therotational axis R of the tool 2. The movements follow circular tracks 11essentially centered on the center H of the initial cavity C, andpresenting suitable differences in radiuses, whereby an orbit of thecenter position of the tool 2 following one circular track is followedby a step 12 outwards to a larger circular track.

In the step described in with reference to FIG. 7, at each orbit of thetool 2, material is removed mainly by the grinding surface 4, (see FIG.1). Depending on the size and rotational speed of the tool 2, and thetype of dental restoration material used, a suitable amount of materialis removed at each orbit of the tool 2.

Of course, regarding the movements of the tool there are a number ofalternatives to the circular tracks separated with radial steps,described with reference to FIG. 7. For example, while moving the tool 2essentially in a plane perpendicular to the rotational axis R, the toolcould, at least at an early phase of the step of removing materialbetween the first and the second level L1, L2, follow a track shaped asa spiral, at which the tool is gradually moved outwards from thestarting point, so that a suitable amount of material is removed at eachorbit of the tool.

The movements of the tool 2 essentially in a plane perpendicular to therotational axis R has the following advantage: Since the grindingsurface 4 of the tool 2 is at a radial distance from the rotational axisR, and since the grinding surface 4, due to the lateral movement of thetool, takes part in the material removing process, it is accomplishedthat essentially all of the working grinding surface of the tool 2 has ahigh velocity. This results in a high material removal rate. The natureof dental restoration materials, i.e. dental ceramic materials, includesa relatively small elastic deformation and essentially no plasticdeformation before a breaking stress of the material is reached. As aresult, if some working surfaces of the tool is moving due to therotation with a relatively slow velocity, the translational movement ofthe tool combined with a relatively small material removal rate, couldcause deformations in the dental restoration material followed by afailure when the ultimate stress has been reached. The high velocity ofthe working grinding surface of the tool 2 accomplished by the inventionwill drastically reduce the risk of failure in the dental restorationmaterial.

In general, the method according to the invention drastically reducesthe risk of damages on materials or tools by making it possible toprevent the cutting depths from becoming too large, (discussed closerbelow), to avoid or minimise movements mainly in the axial direction ofthe tool, and to prevent a contact surface between the tool and thematerial from becoming too large, (discussed closer below).

Referring to FIG. 7, the tool center boundary curve TCBC has anirregular shape. Referring to FIG. 8 a, the movements of the tool islimited outwards by the TCBC, and tool paths in the area enclosed by thetool center boundary curve TCBC are determined in the following way: Acurve, in this example the circle C9, is offset outwards to form toolpaths C10, C11 outside this curve C9, which paths has shapescorresponding to the shape of said curve C9. Instead of offsetting froma circle C9, the outwards offsetting could be made from any suitablecurve, with any shape. As a further alternative, the tool paths can bedetermined as outwardly offsetting curves of a predetermined shape, forexample circles or circle segments, from a point with a suitablelocation.

The distance between the offset curves C10, C11 corresponds to asuitable radial cutting depth of the tool 2.

Curves created by outwards offsetting can intersect the tool centerboundary curve TCBC. Additional outer tool paths are created by outwardsoffsetting, until created curves do not intersect the tool centerboundary curve TCBC, i.e. are located outside the latter.

Where needed, the offset curves are trimmed against the tool centerboundary curve TCBC, removing curve parts outside the latter, so thatsegments C10, C11 of closed curves or circles are created. Suchsegments, or clusters of segments, form sections S1, S2, S3 of theprocessing region, which sections are formed in pockets inside the toolcenter boundary curve TCBC, where the latter presents a more abruptcurvature than the curves C10, C11 created by outwards offsetting. Eachsection, (for example S1 in FIG. 8 a), can present subsections S1-1,S1-2 which are smaller sections or pockets, each with their own curvesegments.

Preferably, in each section S1, S2, S3, (pocket inside the TCBC), and ineach subsection S1-1, S1-2, the curve segments C10, C11 areinterconnected to form a continuous tool path. Preferably, theinterconnection between the segments are formed by interconnectingsegments of the TCBC, or by linear segments taking into account asuitable clearance towards the TCBC. Thus, when removing material, thetool paths within a section S1, S2, S3, or a subsection, S1-1, S1-2, arefollowed successively to minimise the number of tool lifting measuresbetween different sections or pockets S1, S2, S3. This will reduce theprocessing time.

A precision cut following the tool center boundary curve TCBC is made toclean the contour.

An advantage with the technique of determining tool paths by offsettingoutwards a curve, and trimming offset curves against a boundary curveTCBC is that the cutting depth of the tool can be controlled.

Another advantage is that it is easy to check if the tool paths resultin extraordinary movements that are undesired from a material processingpoint of view, e.g. due to a risk of damaging the material or the tool.For example, referring to FIGS. 8 b and 8 c, such a case can arise whena tool path stretches into a “shaded” area, e.g. behind a “peninsula” 21or an island 22 formed by the tool center boundary curve TCBC. Such ashaded tool path is marked with “SX” in FIGS. 8 b and 8 c. Sincematerial has not been removed inside of the shaded tool path the contactsurface of the material and the tool becomes very large. The appearanceof the shaded path as such is easy to detect, when using the techniqueof offsetting a curve outwards.

Preferably, if a tool path is found to be undesired according topredetermined requirements, e.g. regarding the size of the contactsurface of the material and the tool, the tool path is rejected.Preferably, a region 23 is defined including an area covered by therejected tool path SX, and a set of curved, preferably part-circular,tool paths 24 are defined with a suitable center of curvature andradiuses. Alternatively, such tool paths 24 can be straight.

FIG. 9 shows, in a view of the dental material piece 1 sectioned as inFIGS. 1 and 4, a result of the step described above, to remove materialbetween the first and the second level L1, L2, in the form of a cavity15. It can be seen that material has been removed approximately up tothe contour 6 of the intended final cavity, at the second level L2. Itcan be seen that a portion 16 of the dental material piece 1, outsidethe cavity 15, and between the cavity 15 and the contour 6 remains to beremoved. Preferably, this is done by introducing a number of sublevelsbetween the first and second levels L1, L2, and, starting from thelowest sublevel and raising the tool in a stepwise manner, removingmaterial at each sublevel. Similar to what was described above withreference to FIGS. 5 and 6, at each sublevel, i-1, i-2, i-3, a toolcenter boundary curve, TCBCi-1, TCBCi-2, TCBCi-3, is determined. At eachsublevel, for example on sublevel i-2, tool paths are created byoffsetting outwards the tool center boundary curve TCBCi-1 from thesublevel below, i-1, towards the tool center boundary curve TCBCi-2 atthe sublevel i-2. The result is shown in FIG. 10.

In this example, the processing of the dental restoration continues withsimilar steps as those described above. Referring to FIG. 11, in a stepcorresponding to the step described above with reference to FIGS. 2, 3,and 4, material is removed from the dental material piece 1 from a firstlevel L1 of the dental material piece 1 to a second level L2 thereof,the first and the second level L1, L2 being separated by a distance d2in a direction parallel to the rotational axis of the tool. In thisexample, the first level L1 in the step described with reference to FIG.11, is the same as the second level L2 in the step described withreference to FIG. 4.

According to the invention, in a subsequent step, material is removedbetween the first and the second level L1, L2 by moving the tool 2 whilein rotation essentially in a plane perpendicular to the rotational axisR of the tool 2, the result of which is shown in FIG. 12. This is donein the same manner as described above with reference to FIGS. 7 and 8 a.Similar to what has been described with reference to FIG. 9, it can beseen that a portion 16 of the dental material piece 1, outside thecavity 15, and between the cavity 15 and the contour 6 remains to beremoved. In the same manner as described above with reference to FIGS. 9and 10, this is done by introducing a number of sublevels between thefirst and second levels L1, L2, and, starting from the lowest subleveland raising the tool in a stepwise manner, removing material at eachsublevel. The result is shown in FIG. 13.

Referring to FIG. 14, continuing the processing of the dentalrestoration with similar steps as those described above, in a stepaccording to the invention, material is removed from the dental materialpiece 1 from a first level L1 of the dental material piece 1 to a secondlevel L2 thereof. In this example, the first level L1 in the stepdescribed with reference to FIG. 14, is the same as the second level L2in the step described with reference to FIG. 11.

Similar as described above with reference to FIGS. 7 and 8 a, in asubsequent step, material is removed between the first and the secondlevel L1, L2 by moving the tool while in rotation essentially in a planeperpendicular to the rotational axis R of the tool 2, the result ofwhich is shown in FIG. 15. FIG. 16 shows the result of removing aportion 16, shown in FIG. 15, outside the cavity 15, and between thecavity 15 and the contour 6.

Preferably, the lowest level for using the tool 2, used in the stepsdescribed above, is a level that permits creating an initial cavity C ofa predetermined minimum diameter, so that it is ensured, during thecreation of the initial cavity C, that the direction of the movement ofthe tool 2 forms an angle α to a rotational axis R of the tool.

FIG. 17 shows the dental material piece 1 after removing furthermaterial in a similar manner to what has been described above, whereby acavity 15 is obtained. A portion 17 at the bottom of the cavity 15 canbe removed by a suitable tool.

Levels processed by the relatively large tool 2 are analysed regardingareas not processed due to the size of the tool 2. Preferably, thisanalysis is performed from the bottom and up. At each level an innercurve is determined based on the processed area. The inner curve isexpanded outwards similarly to what has been described above withreference to FIG. 8 a, to create tool paths to remove remaining areas.This is done with a suitable tool with smaller dimensions.

Above, the cavity in the dental material piece 1 has been described asbeing created by two steps being repeated alternately, namely: providingan initial cavity C in the material dental piece 1, and removingmaterial outside the initial cavity C. It should be noted that thesesteps can be carried out using the same or different tools.

Alternatively, a step of providing an initial cavity C in the materialdental piece 1 can be followed by repeated steps of removing materialoutside the initial cavity C, whereby the initial cavity C is relativelydeep and material is removed outside of the initial cavity at aplurality of levels. Thereby, initial cavities or pre-cavities, can bepre-made in dental restoration blanks or work pieces. In such a case,the initial cavity C can advantageously be formed before sintering ofthe material, or, when compression moulding the blanks, the initialcavity C can be formed by providing a protruding part in the mould.

1. A method for preparing a dental restoration with at least onerotating material removing tool, comprising providing a dental materialpiece from which the dental restoration is to be prepared, providing aninitial cavity in the material dental piece, and removing materialoutside the initial cavity by moving the tool essentially in a planeperpendicular to the rotational axis of the tool.
 2. A method accordingto claim 1, wherein providing an initial cavity includes moving the toolso that the direction of the movement of the tool forms an angle to therotational axis of the tool.
 3. A method according to claim 2, whereinthe angle is between 80 and 89.5 degrees.
 4. A method according to claim3, wherein, in the step of providing an initial cavity the tool path, asprojected in a plane perpendicular to the rotational axis of the tool,forms a closed loop.
 5. A method according to claim 4, wherein the toolfollows a helical path.
 6. A method according to claim 5, furthercomprising: determining a tool center boundary curve, which representsthe outer limit of the movements of the rotational axis of the tool, ata plane being perpendicular to the axis of rotation of the tool, basedon the intended final cavity surface in a region in the vicinity of anintersection between said intended final cavity surface and said planebeing perpendicular to the axis of rotation of the tool, and the shapeof the tool on at least a part thereof.
 7. A method according to claim6, comprising determining a tool center curve at each level, of aplurality of levels, at, under and/or above said plane beingperpendicular to the axis of rotation of the tool, by offsetting inwardsthe intersection between the intended final cavity surface and therespective level by an amount corresponding to the radius of the tool atthe respective level and determining the tool center boundary curve asthe most inwardly located of the tool center curves.
 8. A methodaccording to claim 7, wherein the location, in a plane perpendicular tothe rotational axis of the tool, of a center of the initial cavity isdetermined as the location of the center of the largest circle that canbe fitted within a boundary curve in a plane perpendicular to the axisof rotation of the tool.
 9. A method according to claim 8, wherein saidboundary curve is the tool center boundary curve.
 10. A method accordingto claim 8, wherein at least one tool path is determined by creating atleast one offset curve by offsetting outwards a curve, and trimming theat least one offset curve against a tool boundary curve.
 11. A methodaccording to claim 10, wherein, if a tool path is found to be undesiredaccording to predetermined requirements, the tool path is at leastpartly rejected, and at least one tool path is defined with a center ofcurvature differing from that of the rejected tool path.
 12. A methodaccording to claim 10, wherein at least one of the tool paths iscircular or part-circular.
 13. A method according to claim 11, whereinat least one of the tool paths is circular or part-circular.
 14. Amethod according to claim 2, wherein, in the step of providing aninitial cavity, the tool path, as projected in a plane perpendicular tothe rotational axis of the tool, forms a closed loop.
 15. A methodaccording to claim 14, wherein the tool follows a helical path.
 16. Amethod according to claim 15, further comprising: determining a toolcenter boundary curve, which represents the outer limit of the movementsof the rotational axis of the tool, at a plane being perpendicular tothe axis of rotation of the tool, based on the intended final cavitysurface in a region in the vicinity of an intersection between saidintended final cavity surface and said plane being perpendicular to theaxis of rotation of the tool, and the shape of the tool on at least apart thereof.
 17. A method according to claim 16, comprising determininga tool center curve at each level, of a plurality of levels, at, underand/or above said plane being perpendicular to the axis of rotation ofthe tool, by offsetting inwards the intersection between the intendedfinal cavity surface and the respective level by an amount correspondingto the radius of the tool at the respective level, and determining thetool center boundary curve as the most inwardly located of the toolcenter curves.
 18. A method according to claim 17, wherein the location,in a plane perpendicular to the rotational axis of the tool, of a centerof the initial cavity is determined as the location of the center of thelargest circle that can be fitted within a boundary curve in a planeperpendicular to the axis of rotation of the tool.
 19. A method A methodfor preparing a dental restoration in a dental material piece with atleast one rotating material removing tool, comprising: providing aninitial cavity in the material dental piece, and removing materialoutside the initial cavity by moving the tool essentially in a planeperpendicular to the rotational axis of the tool, and determining atleast one tool path by creating at least one offset curve by offsettingoutwards a curve and trimming the at least one offset curve against atool boundary curve.
 20. A method according to claim 19, furthercomprising: determining a tool center curve at each level, of aplurality of levels, at, under and/or above said plane beingperpendicular to the axis of rotation of the tool, by offsetting inwardsthe intersection between the intended final cavity surface and therespective level by an amount corresponding to the radius of the tool atthe respective level, and determining the tool center boundary curve asthe most inwardly located of the tool center curves wherein thelocation, in a plane perpendicular to the rotational axis of the tool,of a center of the initial cavity is determined as the location of thecenter of the largest circle that can be fitted within a boundary curvein a plane perpendicular to the axis of rotation of the tool.